Control Engineering Online Update for April 14, 2006

By Control Engineering Staff

04/14/2006

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April 14, 2006

Highlights

Sponsored by Invensys/Foxboro

There simply aren’t enough people in a modern cement plant to achieve true process optimization using conventional manual control methods,” said an engineer for Lafarge Group, a worldwide cement manufacturer. So, the company developed an automated, real-time closed-loop supervisory control system. The expert system application combines real-time reasoning technologies with knowledge from Lafarge engineers to reduce the needed number of human interventions and increase production.

Manufacturing plants are becoming more automated than ever before, dramatically reducing the number of people that run them. But at the same time, operations have been growing far more complex. The result is that operators have so many tasks to perform that it has become more and more difficult for them to manually maintain optimized process conditions.

Enter the rule-engine application or expert system. These systems can deliver production increases and quality improvements despite the shrinking workforce. A rule-engine application can stabilize a process at or near optimal levels by making frequent but small adjustments under standard conditions.

In a cement plant, for example, sensors in the kiln determine operating temperatures, and chemical analysis of exhaust gases shows whether overburning or underburning is occurring. Based on this type of information, operators are constantly expected to make decisions regarding the feed rate of fuel and raw material into the kiln. But operators have so many tasks to perform that, by the time they are free to address this type of issue, a major swing can occur that requires a large step to bring the process back under control. This creates variation in the quality of the finished product and also runs the risk of moving the process too far in the opposite direction—in the worst case, causing an upset that may result in a production loss.

Lafarge Group, Paris, France, the world's largest cement producer, overcame this problem and in the process increased production by 4% to 5% in 50 cement mills. They did this by implementing a rule-engine application that deploys the knowledge of Lafarge's experts to optimize operating parameters such as feed rates. The application makes smaller and more frequent changes than operators were able to do, increasing output consistency and making it possible to run closer to operating limits.

The application—called "Lucie," for Lafarge Universal Cement Inference Engine—was developed by an in-house team based on the G2 real-time rule-engine platform from Gensym Corporation, Burlington, Mass. A senior engineer with the team said the expert system operates mills and kilns in closed loop mode for approximately 90% of the time, while operators handle the most complicated situations that make up the other 10%.

"There simply aren't enough people in a modern cement plant to achieve true process optimization using conventional manual control methods," the engineer said. "Our goal in developing Lucie was to provide an automated method of optimizing the process during the vast majority of the time when the plant is operating under standard conditions. Very complex situations, which only occur occasionally, are very difficult to parameterize so we feel they are better handled by the operators."

The cement-making process

Lucie has grown over the years to become a reliable and stable means of controlling most of the operating equipment in a cement plant. The cement manufacturing process begins when limestone is quarried and crushed. The material is finished-ground and then mixed with other materials. Then it is burned in a kiln in a process that breaks calcium carbonate down into hard nodules of calcium oxide called "clinkers." The kiln is the area where capital costs are highest, fuel demands are largest, and process control is the most crucial. The clinkers are milled to produce a fine grey powder that is blended with gypsum and other materials to produce finished cement. The large electric power demands of finish milling mean that improving the efficiency of grinding and separation of the ground clinker can yield significant energy efficiency improvements and cost savings.

G2 applications transform operations data into automated decisions and actions in real time. The G2 platform combines real-time reasoning technologies, including rules, procedures, object modeling, simulation, and graphics, into a single development and deployment environment. The platform supports the entire application lifecycle, and Lafarge is continually interviewing engineers and operators to improve the Lucie system. A graphical rule language was developed to simplify the entry of knowledge.

Lucie control strategies are based on closed loop sets of calculations that are kept generic to facilitate maintenance. The generic software release that forms the basis of each plant's control system provides strategies for different types of kilns, grate coolers, conventional ball mills, and vertical spindle mills. A fuel manager manages the feeding of alternate energy sources to the kiln system.

Lucie collects sensor signals from the process controllers. Sensor signals are subjected to validity and trustworthiness tests and signals that fail the tests may be subjected to more rigorous filtering or thrown out. Treated sensor values are mathematically processed to generate short term actions based on the goals of increasing production rates, improving product quality, and reducing energy consumption. Short-term actions are modeled based on the expertise of the company's most experienced operators. For example, abrupt action is taken to recover from an upset condition. After conditions have returned to normal, these actions are reversed to maintain long-term stability .

Too hot, too cold, or just right?

Lucie has judgment functions that apply expertise using a reasoning technique known as fuzzy logic to judge what the process conditions are in real time. For example, an oven judgment function might conclude whether it is too cold, too hot, or just right. Or using measurements for oven temperature, kiln torque, and gas analysis, Lucie will judge if the material is at correct burn or quality level. With these judgments, there is no need to wait for lab results on quality, which can take a half hour to run and report back to operations. With Lucie, the goal is to act immediately based on the judgments.

Expert system implementation

Lafarge is working to implement Lucie in plants around the world. The implementation process begins with a workshop that teaches plant personnel how to apply Lucie to finish mills, which are relatively easy to control. An engineer or information technology person is assigned the job of connecting Lucie to the plant's sensors and control systems. Once the expert system is up and running, this person can go back to their normal job. After the plant has experience with running Lucie to control the finish mill, they begin using the expert system to run other areas, such as kilns. Using this basic process, Lucie has been implemented in 50 out of Lafarge's 120 kilns around the world.

The benefits of the system to Lafarge are clear. Since the company has the most experience with closed loop control of finish mills, this is also where the benefits are most easily quantified. A consistent approach to mill circuit control has increased mill production by an average of 6% in U.S. plants. The variability of quality was also reduced by 16%. The first mill where Lucie was installed in Greece recorded an immediate increase in production of 2.3% compared to the others that were still operating manually.

"If the plant is sold out, these production increases can immediately be converted to revenues," the Lafarge engineer said. "If not, mills can still improve quality and reduce operating costs." For example, two cement mills in Malaysia operating below full capacity were able to reduce power consumption by 4.1% and 5.1% and reduce quality variability by 40%.

Some Lafarge plants are also starting to see improvements in the more difficult kiln area. In one plant, kiln production quickly climbed to record levels after Lucie was implemented. A study of kiln control in France showed that Lucie was able to reduce clinker quality variation by nearly 40%. This study also showed that the number of human interventions per hour dropped from 5.83 to 0.249 after installing Lucie, providing operators with much more time to focus on proactive tasks.

"Our plants throughout the world that have implemented Lucie have achieved production increases, quality improvements, and operating cost reductions," the engineer concluded. "Having seen the benefits that can be achieved, we are working as quickly as possible to implement the expert system in our remaining plants in order to maximize the competitive advantage that we are achieving from real-time closed-loop supervisory control."